Treatments of disorders using nucleic acid drugs include antisense therapies, antigene therapies, aptamers, siRNAs, and the like. An antisense therapy is the procedure for treatment or prevention of diseases involving inhibiting a translation process of pathogenic RNAs by externally introducing oligonucleotides (antisense strands) complementary to disease-associated mRNAs to form the double strands. The mechanism of siRNAs is similar to that of the antisense therapies, which involves inhibiting translation from mRNAs to proteins by administration of double-stranded RNAs to the body. Meanwhile, in the antigene therapies, transcription of DNA to RNA is suppressed by externally introducing triple-strand-forming oligonucleotides corresponding to the DNA sites transcribed into the pathogenic RNA. Aptamers, which are small nucleic acid molecules (oligonucleotides), exert their functions by binding to disease-related biological components, such as proteins.
Although various artificial nucleic acids have been developed as materials for such nucleic acid drugs, there has not been found any ideal molecule yet. For example, the materials developed for nucleic acid drugs to date include S-oligo (phosphorothioate), 2′,4′-BNA (bridged nucleic acid)/LNA (locked nucleic acid) (See Patent Documents 1 to 3 and Non-patent Documents 1 to 4). S-oligo is commercially available as an antisense drug for cytomegalovirus in United States. While this drug has high nuclease resistance, it has a weakness to be improved concerning about its low binding affinity for the target nucleic acid strands. All types of 2′,4′-BNA/LNA which have ever been developed have high binding affinities for their target nucleic acid strands and they are the most promising molecules as the materials for the future nucleic acid drugs. However, they still remain to be improved with regard to their nuclease resistance which is not enough to be stable in vivo.
Furthermore, in recent years, application of oligonucleotides having nucleoside structures such as those represented by formulas a and b below:

to the aforementioned materials has also been proposed (Patent Document 4).
However, extremely complicated processes are required for production of nucleosides themselves serving as a basis for structures such as those represented by the formulas a and b above. For this reason, development of oligonucleotides having equivalent or superior performance to such oligonucleotides and also having much higher industrial production efficiency is demanded.